Report the users who bump the old threads to their signature campaign managers. Because most of such bumps are done with the only purpose to gain "post count" (unless we talk about genuine newbies). And in such cases warning and/or kick out of signature campaign because of spam will make them learn.
I don't know if it's too harsh, but .. well.. it's an idea....
There was an annoying newbie spammer yesterday that was trying to bump up his post count pretty quickly by replying to year old threads just with variants of "I'm interested" etc and then about 5-10 other Sig spammers and farmers would post something of the same and it's very time consuming to delete all the responses.
It is annoying and some people dislikes it so they call the necro guy to stop doing it and call the mods to lock that topic
In the other hand in my opinion the necro newbie is just an alt of a sig guy who try to increase his posts so he may answer just after the necro or wait few posts after few members to avoid being busted
Hilariousandco idea is great and what botany said is logic and i asked the same question months ago but what i suggest in the meantime is to delete the spammy posts after the necro one without exception and manually lock the topic until the previous idea come to life
If someone is trying to open a real discussion about the same locked topic he can open a new one then mention the locked one in his post or copy the text and put it on a quote.